Polyester resin films are widely used in various technological fields by virtue of their excellent mechanical, electrical and chemical-resistance properties.
In particular, biaxially-stretched films of polyethylene terephthalate are superior to other films both in terms of dimensional stability and in terms of tensile properties, particularly in view of their high modulus of elasticity.
However, polyester films have drawbacks, mainly due to their very high relative density and to the fact that applications in the field of information technology, such as for example for electronic whiteboards and similar devices require them to be highly loaded with white pigments in order to be sufficiently opaque. Various methods for producing foamed films or sheets of polyester resin are known.
Thick low-density foamed materials made of polyester resin, due to their high thermal insulation properties, which prevent effective cooling of the internal parts of the materials as they exit from the extruders, have a relatively high crystallinity which is difficult to reduce.
No solution has been found so far to the problem of being able to mono- or biaxially stretch foamed sheets made of polyester resin, which have a density of less than 600-700 kg/m3 and have crystallinity or are crystallizable.
The main difficulty encountered in the mono- and biaxial stretching of said low-density foamed sheets consists in the possibility of their breaking during stretching.
It is known from WO 97/33948 to produce labels from polyester foamed films which may also be mono or biaxially stretched when obtained from amorphous resins such as poly(1,4-dimethylolcyclohexile)terephthate or amorphous copolyethylene terephthate-isophthalate.
The possibility to have mono- and biaxially-stretched polyester-resin foamed sheets or films having low apparent density and sufficiently high crystallinity might offer considerable advantage, especially in the view of the improved mechanical properties that said sheets and films might have.
It has now been found unexpectedly that it is possible to mono- and biaxially stretch, without rupture problems or other drawbacks, foamed aromatic polyester resin sheets and films having a bulk density of less than 700 kg/m3, preferably less than 400 kg/m3, in which the resin has a crystallization rate such that by heating for 10 minutes at 120xc2x0 C. the crystallinity can reach values as high as 30-35%, and to obtain stretched sheets or films having a relatively low apparent density which have high mechanical properties, particularly in terms of high modulus and high impact resistance and good opacity or translucence associated with sparkling reflectance properties.
Preferably, the crystallinity that can be developed by heating at 120xc2x0 C. for 10 minutes is from 5 to 35%.
The high impact resistance of the resulting stretched sheets or films is surprising being considerably higher than that of the sheets and films before stretching.
It has been found that the mono- and biaxial stretching of foamed sheets having the above indicated thickness, crystallinity and density characteristics is feasible if said sheets are obtained from polyester resin having sufficiently high melt strength and melt viscosity values which are higher than certain given limit values.
The melt strength of the usable resin is at least 1 centinewton at 280xc2x0 C. and melt viscosity is at least 1500 Pa.s at 280xc2x0 C. with a shear rate which tends to zero.
Melt strengths of 10 to 150 or more centinewtons and melt viscosities of 2,000-20,000 Pa.s can be used conveniently.
The melt strength measured on the resin forming the foamed sheets or films presents value lower than those of the resin used in for preparing the sheets and films.
The intrinsic viscosity is generally between 0.8 and 1.5 dl/g.
The above specified rheological properties refer to the resin before it is subjected to the extrusion-foaming process, but they can be acquired during said process.
The aromatic polyester resins usable to obtain the resins having the above specified rheological properties are prepared by polycondensation, according to known methods, of dicarboxylic aromatic acids with diols containing 2 to 12 carbon atoms or by transesterification of lower alkyl esters of dicarboxylic acids with diols having 2-12 carbon atoms and subsequent polycondensation of the diol esters.
Terephthalic acid, isophthalic acid and naphthalene dicarboxylic acids are preferred aromatic acids.
Polyethylene terephthalate and copolymers thereof in which 1 and up to 20-25 and preferably 1-25 monomeric units derived from terephthalic acid are substituted with units derived from isophthalic acid and/or naphthalene dicarboxylic acids are preferred resins.
The polyester resins having the above specified rheological properties can preferably be obtained by solid-state polycondensation (SSP) of polyester resins having an intrinsic viscosity of less than about 0.7 dl/g added with a dianhydride of a preferably aromatic tetracarboxylic acid, particularly pyromellitic dianhydride, in an amount of 0.05 to 2% by weight, working under such temperature conditions and with such durations as to increase the melt strength and the melt viscosity of the resin to the chosen values.
The intrinsic viscosity of the resin after SSP is generally increased to values of more than 0.8 dl/g.
The above indicated solid-state polycondensation is performed according to known methods.
A particularly suitable method is described in U.S. Pat. No. 5,243,000, whose description is included herein by reference.
Other methods suitable to obtain the melt strength and melt viscosity values according to the invention are disclosed in U.S. Pat. Nos. 5,288,764 and 5,229,432, whose description also is included by reference.
The polyester resins can be used in mixture with other thermoplastic polymers, particularly with polyamide resins used in an amount from approximately 2 to 50% by weight. Mixtures or alloys of this type are described in WO 94/09069, whose preparation method is included herein by reference.
A polyamide which is particularly suitable especially when one wishes to give improved gas-barrier properties (oxygen and CO2) is poly-m-xylilene adipamide.
This polyamide is mixed while melted with the polyester resin, which is premixed, also while melted, with a dianhydride of a tetracarboxylic aromatic acid, particularly pyromellitic dianhydride, used in an amount from 0.05 to 2% by weight on the polyester resin.
Other polymers that can be used are aliphatic polyester resins obtainable from aliphatic dicarboxylic acids and from diols or from aliphatic hydroxides-acids or from the corresponding lactones or lactides.
Poly-epsilon-propiolactone is a representative resin.
These resins are added in amounts of up to 40% by weight and give biodegradability properties to the resin thus mixed.
Another aspect of the invention is the finding that the addition of amounts between 0.5 and 10% by weight to the polyester resin of an aliphatic or aromatic polyamide with a high or low molecular mass allows to significantly reduce the amount of the unreacted pyromellitic dianhydride present in the stretched and foamed sheets and films and the amount of acetaldehyde.
The foamed sheets suitable for being mono- and biaxially stretched have a bulk density of about 50 to 700 kg/m3. Thickness is generally form 0.5 to 5 mm.
To produce the monoaxially-stretched foamed films with a thickness reduced to approximately 30 microns, the thickness of the starting foamed sheets is from about 0.6 to 2 mm; when instead one wishes to obtain biaxially-stretched sheets, one begins with thicker sheets (2-5 mm).
In the case of biaxial stretching, the bulk density after stretching is increased considerably (even fourfold for 4:1 stretching).
However, when a hydrocarbon is used as foaming agent, the residual hydrocarbon enclosed within the cells expands due to the heating required to bring the sheet or film to the temperature suitable for stretching. It is thus possible to obtain biaxially-stretched sheets or films with a density which is lower than before stretching.
In the case of monoaxial stretching, density is generally reduced (this is due to the different apparatus used in monoaxial stretching with respect to biaxial stretching).
In both cases, one works so as to have a density of preferably less than 500 kg/m3 after stretching.
The average size of the cells in the starting foamed materials can vary from 0.01 to 1 mm according to the conditions used in the extrusion foaming process, such as for example the type of nucleating agent and foaming agent and the amounts used.
Values of 0.2-0.4 mm are representative.
The average size of the cells in the sheets and films after biaxial stretching is increased with respect to the size before stretching: in monoaxial stretching, the cells are elongated.
Biaxial stretching is performed according to conventional methods, working at temperatures which are higher than the Tg of the polyester resin but lower than the melting point.
Temperatures of 80 to 120xc2x0 C. are suitable: residence times during stretching range from a few seconds to 40 or more.
The crystallinity of the sheet and film before stretching is kept low enough in order to be able to easily perform stretching (preferably lower than 10%).
The biaxial stretch ratio in both directions is generally from 1.5:1 to 5:1 and preferably up to 3:1 and stretching in the two directions can be performed simultaneously or sequentially.
Monoaxial stretching is performed either in the direction of the machine or transversely. The stretch ratio is generally from 1.1:1 to 4:1.
Stretching is generally performed on a series of calendering units heated to 95xc2x0-110xc2x0 C. which rotate at different speeds.
In many cases it is advantageous to subject the biaxially- and monoaxially-stretched material to a heat-stabilization treatment, working at temperatures between for example 160xc2x0 and 220xc2x0 C. for a few seconds (generally 10-120 seconds).
The treatment allows to achieve good dimensional stabilization of the material and improve its mechanical properties.
The heat set material has relatively low heat-shrinkage values with respect to the unstabilized material (less than 5% heat shrinkage at, for example, 180xc2x0 C. for a few minutes).
Crystallinity after stretching is higher than before stretching; it can reach values of 30% and more in the case of stretched sheets and films subjected to heat-setting treatment.
As noted, the mechanical properties of the mono- and biaxially-stretched foamed sheets and films are considerably improved with respect to before stretching.
The modulus of elasticity and impact resistance are particularly high.
For example, in the case of a biaxially-stretched film with a thickness of 40 microns, the modulus can reach 2 GPa or more; the ultimate tensile strength is 50-60 MPa and breaking elongation is 50%.
The opacity of the stretched material is high, generally from 40% to 80% in the case of the biaxially stretched film. The gloss characteristics are improved considerably with respect to the unstretched foamed material. The translucent sheets, particularly those biaxially stretched present a sparkling reflectance involving the whole sheet, which renders the sheets particularly suitable for applications such as the holographic printing.
The mono- and biaxially-stretched sheets or films according to the invention are characterized by a high level of heat-shrinkage. For example, a biaxially-stretched film with a thickness of 0.04 mm and with a stretch ratio of 3:1 exhibits a shrinkage of 20-30% when heated to 180xc2x0 C. for a few minutes. This property is utilized for many applications, particularly for labeling polyester resin bottles in which the label is made to adhere to the bottle by virtue of the shrinkage that said label undergoes when the bottle is heated.
By virtue of their flexibility, breathability and high permeability to water vapor, the mono- and biaxially-stretched sheets or films are used in particular in the textile field.
Their easy inking furthermore makes them particularly suitable in applications for replacing paper or cardboard.
The mono- and biaxially-stretched sheets and films can furthermore be included in multilayer materials in which a layer constituted for example by polyester resin reinforced with glass fibers or by a low-melting polyester resin or by other materials is adjacent on one or both sides of the stretched sheet or film.
The low-melting polyester is generally a copolyethylene terephthalate/isophthalate which contains more than 7% in moles of isophthalic acid units.
The multilayer material can be prepared both by coextruding from a battery of extruders the resin to be foamed and the resin or resins that form the adjacent layers and by then stretching the resulting multilayer material, or by gluing one or more layer of other materials to the foamed sheet or film or by making them adhere to said sheet or film.
Finally, the high opacity of the biaxially-stretched foamed films and sheets and their high-level mechanical properties make them particularly suitable for applications in information technology and photography.
Other applications not mentioned here can be evident to the skilled person.